Timing-based servo (TBS) is a technology developed specifically for linear tape drives in the late 1990s. In TBS systems, recorded servo patterns include transitions with two different azimuthal slopes. Head lateral position is derived from the relative timing of pulses generated by a servo reader reading the servo patterns. The complete format for the linear tape-open (LTO) drives of generation 1 (LTO-1), including the geometry of the servo patterns, was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 4 (LTO-2 to LTO-4), where the servo format was not modified from LTO-1, can be found on the World Wide Web at http://www.ultrium.com. The servo format remains unchanged also for LTO drives of generation 5 (LTO-5). Furthermore, TBS patterns allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position. Servo patterns comprise servo frames, whereby each servo frame has a length equal to 200 microns and encodes one bit of LPOS information.
In the current linear tape-open (LTO) servo format, the servo patterns are pre-recorded in five bands distributed across the tape. Data is recorded in the four regions located between pairs of servo bands. The positioning of the five servo bands and the four data bands on a tape 300 is specified in the LTO format, as represented in prior art FIG. 3. In read/write head modules 302 of LTO tape drives, two servo readers 304, 306 are normally available per head module, from which LPOS information, as well as PES, can be derived. The identity of each servo band (n), 0≦n≦4, is determined by the relative positions down the tape of frames in servo bands n and n+1, reading with the top and bottom servo readers, respectively, where the head module is assumed perpendicular to the tape edge. The four possible relative shifts of the patterns in the servo band n+1 with respect to the patterns in the servo band n when the tape is moving in the forward direction (BOT to EOT) are equal to +/−33 microns and ±/−66 microns in the LTO format.
Optimum detection of the servo patterns is achieved by a synchronous servo channel employing a matched-filter interpolator/correlator, which ensures that filtering of a servo reader signal is performed not only at constant tape velocity, but also during acceleration and deceleration. A synchronous servo channel thus ensures an optimum processing of a servo signal for the generation of lateral position and velocity estimates, which are then employed for the control of track-following and reel-to-reel servomechanisms of the tape drive. The initial parameters that are used for proper servo channel operation, e.g., tape velocity and head lateral position, are estimated from the peak locations of the servo channel signal, using the knowledge of the UFO servo pattern geometry.
In drives using flanged rollers to transport the tape between the reels, the flanges limit the motion of the tape thereby increasing the probability that it is in a correct lateral position, and that the tape-to-head skew is small, but also introduce debris that accumulates on the tape, impacts the lifetime of the tape, and creates undesirable dynamic effects. One solution to this problem is to remove the flanges, as described in A. J. Argumedo, et al., “Scaling tape-recording areal densities to 100 Gb/in2,” IBM J. Res. & Dev., Vol. 52. no. 4/5, July/Sept 2008.
By removing the flanges, there is no constraint on the lateral motion of the tape. Consequently, lateral tape motion (LTM) is more pronounced, leading to large tape-to-head skew. Compensation with skew-following actuation is thus used to keep the head perpendicular to the tape edge. For this purpose, a new actuator may be used in the next generation of tape drives that will include a rotational degree of freedom. This further degree of freedom introduces a large uncertainty on the initial value of the tape-to-head skew, which cannot be resolved by reading the servo information unless the servo bands are identified.
Because of the four possible relative shifts between servo bands, however, it is difficult to simultaneously identify the tape-to-head skew and which servo bands are being read by the servo readers on the head module. The need therefore exists for a robust and reliable method for identifying the servo bands spanned by the head module and for determining the tape-to-head skew. This need exists not only in LTO tape systems, but other tape formats which include servo patterns for use in aligning the tape with the head reading and/or writing information from/to the tape.